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    SubjectRe: [PATCH RFC tip/core/rcu 4/5] srcu: Provide polling interfaces for Tiny SRCU grace periods
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    Hi Paul,

    On 11/17/2020 6:10 AM, paulmck@kernel.org wrote:
    > From: "Paul E. McKenney" <paulmck@kernel.org>
    >
    > There is a need for a polling interface for SRCU grace
    > periods, so this commit supplies get_state_synchronize_srcu(),
    > start_poll_synchronize_srcu(), and poll_state_synchronize_srcu() for this
    > purpose. The first can be used if future grace periods are inevitable
    > (perhaps due to a later call_srcu() invocation), the second if future
    > grace periods might not otherwise happen, and the third to check if a
    > grace period has elapsed since the corresponding call to either of the
    > first two.
    >
    > As with get_state_synchronize_rcu() and cond_synchronize_rcu(),
    > the return value from either get_state_synchronize_srcu() or
    > start_poll_synchronize_srcu() must be passed in to a later call to
    > poll_state_synchronize_srcu().
    >
    > Link: https://lore.kernel.org/rcu/20201112201547.GF3365678@moria.home.lan/
    > Reported-by: Kent Overstreet <kent.overstreet@gmail.com>
    > [ paulmck: Add EXPORT_SYMBOL_GPL() per kernel test robot feedback. ]
    > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
    > ---
    > include/linux/rcupdate.h | 2 ++
    > include/linux/srcu.h | 3 +++
    > include/linux/srcutiny.h | 1 +
    > kernel/rcu/srcutiny.c | 52 ++++++++++++++++++++++++++++++++++++++++++++++--
    > 4 files changed, 56 insertions(+), 2 deletions(-)
    >
    > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
    > index de08264..e09c0d8 100644
    > --- a/include/linux/rcupdate.h
    > +++ b/include/linux/rcupdate.h
    > @@ -33,6 +33,8 @@
    > #define ULONG_CMP_GE(a, b) (ULONG_MAX / 2 >= (a) - (b))
    > #define ULONG_CMP_LT(a, b) (ULONG_MAX / 2 < (a) - (b))
    > #define ulong2long(a) (*(long *)(&(a)))
    > +#define USHORT_CMP_GE(a, b) (USHRT_MAX / 2 >= (unsigned short)((a) - (b)))
    > +#define USHORT_CMP_LT(a, b) (USHRT_MAX / 2 < (unsigned short)((a) - (b)))
    >
    > /* Exported common interfaces */
    > void call_rcu(struct rcu_head *head, rcu_callback_t func);
    > diff --git a/include/linux/srcu.h b/include/linux/srcu.h
    > index e432cc9..a0895bb 100644
    > --- a/include/linux/srcu.h
    > +++ b/include/linux/srcu.h
    > @@ -60,6 +60,9 @@ void cleanup_srcu_struct(struct srcu_struct *ssp);
    > int __srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp);
    > void __srcu_read_unlock(struct srcu_struct *ssp, int idx) __releases(ssp);
    > void synchronize_srcu(struct srcu_struct *ssp);
    > +unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp);
    > +unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp);
    > +bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie);
    >
    > #ifdef CONFIG_DEBUG_LOCK_ALLOC
    >
    > diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h
    > index fed4a2d..e9bd6fb 100644
    > --- a/include/linux/srcutiny.h
    > +++ b/include/linux/srcutiny.h
    > @@ -16,6 +16,7 @@
    > struct srcu_struct {
    > short srcu_lock_nesting[2]; /* srcu_read_lock() nesting depth. */
    > unsigned short srcu_idx; /* Current reader array element in bit 0x2. */
    > + unsigned short srcu_idx_max; /* Furthest future srcu_idx request. */
    > u8 srcu_gp_running; /* GP workqueue running? */
    > u8 srcu_gp_waiting; /* GP waiting for readers? */
    > struct swait_queue_head srcu_wq;
    > diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
    > index 3bac1db..b405811 100644
    > --- a/kernel/rcu/srcutiny.c
    > +++ b/kernel/rcu/srcutiny.c
    > @@ -34,6 +34,7 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp)
    > ssp->srcu_gp_running = false;
    > ssp->srcu_gp_waiting = false;
    > ssp->srcu_idx = 0;
    > + ssp->srcu_idx_max = 0;
    > INIT_WORK(&ssp->srcu_work, srcu_drive_gp);
    > INIT_LIST_HEAD(&ssp->srcu_work.entry);
    > return 0;
    > @@ -114,7 +115,7 @@ void srcu_drive_gp(struct work_struct *wp)
    > struct srcu_struct *ssp;
    >
    > ssp = container_of(wp, struct srcu_struct, srcu_work);
    > - if (ssp->srcu_gp_running || !READ_ONCE(ssp->srcu_cb_head))
    > + if (ssp->srcu_gp_running || USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
    > return; /* Already running or nothing to do. */
    >
    > /* Remove recently arrived callbacks and wait for readers. */
    > @@ -147,14 +148,19 @@ void srcu_drive_gp(struct work_struct *wp)
    > * straighten that out.
    > */
    > WRITE_ONCE(ssp->srcu_gp_running, false);
    > - if (READ_ONCE(ssp->srcu_cb_head))
    > + if (USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))

    Should this be USHORT_CMP_LT ?

    > schedule_work(&ssp->srcu_work);
    > }
    > EXPORT_SYMBOL_GPL(srcu_drive_gp);
    >
    > static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
    > {
    > + unsigned short cookie;
    > +
    > if (!READ_ONCE(ssp->srcu_gp_running)) {
    > + cookie = get_state_synchronize_srcu(ssp);
    > + if (USHORT_CMP_LT(READ_ONCE(ssp->srcu_idx_max), cookie))
    > + WRITE_ONCE(ssp->srcu_idx_max, cookie);

    I was thinking of a case which might break with this.

    Consider a scenario, where GP is in progress and kworker is right
    before below point, after executing callbacks:

    void srcu_drive_gp(struct work_struct *wp) {
    <snip>

    while (lh) {
    <cb execution loop>
    }
    >>> CURRENT EXECUTION POINT

    WRITE_ONCE(ssp->srcu_gp_running, false);

    if (USHORT_CMP_LT(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
    schedule_work(&ssp->srcu_work);
    }

    Now, at this instance, srcu_gp_start_if_needed() runs and samples
    srcu_gp_running and returns, without updating srcu_idx_max

    static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
    {
    if (!READ_ONCE(ssp->srcu_gp_running)) returns true
    <snip>
    }

    kworker running srcu_drive_gp() resumes and returns without queueing a
    new schedule_work(&ssp->srcu_work); for new GP?

    Prior to this patch, call_srcu() enqueues a cb before entering
    srcu_gp_start_if_needed(), and srcu_drive_gp() observes this
    queuing, and schedule a work for the new GP, for this scenario.


    WRITE_ONCE(ssp->srcu_gp_running, false);
    - if (READ_ONCE(ssp->srcu_cb_head))
    + if (USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
    schedule_work(&ssp->srcu_work);

    So, should the "cookie" calculation and "srcu_idx_max" setting be moved
    outside of ssp->srcu_gp_running check and maybe return the same cookie
    to caller and use that as the returned cookie from
    start_poll_synchronize_srcu() ?

    srcu_gp_start_if_needed()
    cookie = get_state_synchronize_srcu(ssp);
    if (USHORT_CMP_LT(READ_ONCE(ssp->srcu_idx_max), cookie))
    WRITE_ONCE(ssp->srcu_idx_max, cookie);
    if (!READ_ONCE(ssp->srcu_gp_running)) {
    <snip>
    }


    Thanks
    Neeraj

    > if (likely(srcu_init_done))
    > schedule_work(&ssp->srcu_work);
    > else if (list_empty(&ssp->srcu_work.entry))
    > @@ -196,6 +202,48 @@ void synchronize_srcu(struct srcu_struct *ssp)
    > }
    > EXPORT_SYMBOL_GPL(synchronize_srcu);
    >
    > +/*
    > + * get_state_synchronize_srcu - Provide an end-of-grace-period cookie
    > + */
    > +unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp)
    > +{
    > + unsigned long ret;
    > +
    > + barrier();
    > + ret = (READ_ONCE(ssp->srcu_idx) + 3) & ~0x1;
    > + barrier();
    > + return ret & USHRT_MAX;
    > +}
    > +EXPORT_SYMBOL_GPL(get_state_synchronize_srcu);
    > +
    > +/*
    > + * start_poll_synchronize_srcu - Provide cookie and start grace period
    > + *
    > + * The difference between this and get_state_synchronize_srcu() is that
    > + * this function ensures that the poll_state_synchronize_srcu() will
    > + * eventually return the value true.
    > + */
    > +unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp)
    > +{
    > + unsigned long ret = get_state_synchronize_srcu(ssp);
    > +
    > + srcu_gp_start_if_needed(ssp);
    > + return ret;
    > +}
    > +EXPORT_SYMBOL_GPL(start_poll_synchronize_srcu);
    > +
    > +/*
    > + * poll_state_synchronize_srcu - Has cookie's grace period ended?
    > + */
    > +bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie)
    > +{
    > + bool ret = USHORT_CMP_GE(READ_ONCE(ssp->srcu_idx), cookie);
    > +
    > + barrier();
    > + return ret;
    > +}
    > +EXPORT_SYMBOL_GPL(poll_state_synchronize_srcu);
    > +
    > /* Lockdep diagnostics. */
    > void __init rcu_scheduler_starting(void)
    > {
    >

    --
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